In prior art location tracking systems, a tracking device provides its respective location, e.g., its latitude and longitude, or information from which such location can be determined, to a remote monitoring center. At the monitoring center, or some other associated place, the location of the tracking device is determined, if necessary, and then stored and/or processed. To this end, each tracking device receives signals from global positioning system (“GPS”) satellites and/or wireless signals from terrestrial antennas, hereinafter “other wireless signals”. Each tracking device is typically carried by an entity, hereinafter the “monitored entity”, and there may be many different types of monitored entities, including but not limited to, an individual, a moving vehicle, a product, or a product container. The information stored at the remote monitoring center or some other associated location may be used to provide a history of the location of the tracking device and its associated entity as a function of time.
Each tracking device can be implemented as a unitary device, the so-called “one-piece” tracking device, or as multiple devices that communicate with one another. In either case, each tracking device contains a GPS and/or other wireless signal receiver for respectively receiving GPS signals and/or other wireless signals. Either one or both of these signals may be used to determine the location of the tracking device. Further, GPS and other wireless signals may be used at the same time to determine device location or one signal may be used as a backup when the received strength of the other signal is not sufficient. The determination of the device's location may be performed by the device itself or at a remote location. A “dumb” location tracking device is one that merely retransmits the received GPS and/or other wireless signal to a remote location wherein the location of the tracking device is derived from these received signals. A “smart” location tracking device, on the other hand, possesses the capability of deriving its location from the received GPS or other wireless signals and subsequently transmits its determined location to a remote location. In either case, such transmissions to the remote location are typically periodic to reduce consumption of the tracking device's internal battery, but can be immediate, if desired or if one or more prescribed “alarm” conditions are detected. Alarm conditions include, but are not limited to, detection of tracking device tampering, or a determination that the device is located in a prohibited zone, i.e., an “exclusion zone” or that the device is outside of a permitted zone, i.e., a “inclusion zone”. Such zones can be set individually to match the requirements for the monitored entity. Smart or dumb tracking devices can be “passive”, “active” or a combination thereof. Active location tracking devices communicate their respective location or their received GPS or other wireless signals directly to a remote monitoring station. Passive location tracking devices transmit their respective locations or their respective received GPS or other wireless signals to an intermediary device, such as a docking station, which, in turn, transmits such signals via wired or wireless communications to the remote location. Some location tracking devices may operate so as to be active at certain times and passive at other times.
Tracking devices can be used in a variety of applications in which attempts to interfere with the operation of the location tracking device are made. One such application where this situation arises is where the tracking device along with a remote monitoring center is used to track the location of an “offender”, i.e., an individual who are part of a governmental program, such as parole or the like, wherein monitoring of the offender's location is required. Another application is the tracking of vehicles, such as delivery vehicles. In either application, the location tracking device is affixed to the entity to be monitored and generally can't be removed by other than authorized persons. Further, any attempt by an unauthorized persons to remove the tracking device or to disable its operation results in the transmission of an alarm signal to the remote monitoring station.
While existing tracking devices with these forms of tamper detection capability perform satisfactorily, they are unable to detect more subtle types of tampering which do not leave any permanent visible clues. For example, individuals have learned that the operation of a location tracking device can be thwarted by interfering with the device's ability to receive signals, e.g., GPS and/or other wireless signals, from which the location of the tracking device can be determined. One way of interfering with the signal-receiving capability of the location tracking device is to place signal-shielding material around the tracking device. Another way of accomplishing the same result is to utilize a signal-jamming device, i.e., a device that emits a jamming signal that extends across the frequency band of the GPS and/or other wireless signal from which the location of the tracking device can be determined. Because the signal magnitude of the jamming signal is substantially greater than that of the GPS or other wireless signal, the GPS or other wireless signal is “masked” or equivalently the signal receiver is shielded from properly receiving and processing these signals. The term “shielding” with respect to signals or tampering shall be used in this application to refer to the use of signal-shielding material and/or a signal jamming device to interfere with the operation of a location tracking device.
Signal shielding, if used on a permanent basis, will eventually create a reaction by the monitoring authorities. However, signal shielding is especially troublesome as it may be used temporarily. The shielding can be easily removed after placement about the tracking device and/or the jamming device can be turned off. In either case, there is no visual trace that either of these techniques have been used and there is no way to distinguish between temporary shielding and other non-tampering events, such as a temporary malfunction of the location tracking device or its temporary location in an area where GPS or other wireless signal reception is poor. Further, when shielding is temporarily used, it creates a window of opportunity during which the location of the monitored entity is unknown or not reliably known. Accordingly, it would be desirable if a mechanism could be devised for location tracking devices and systems that would distinguish between signal shielding and other plausible, unintentional non-tampering events.